The second class of mollusks or those that breathe air through the water, have a respiratory cavity instead of a lung, in which is placed a series of leaflets, arranged like the teeth of a comb in two series of lines, forming the so-called gills. The mouth, also, is placed at the end of a long rostrum, or proboscis, and not in the lower plane of the head, as in the first class. Among the most common of this class are the river snails, known as Strepomatids. There are about three hundred species in this family, and with two or three exceptions they are confined entirely to the United States in geographical distribution. The shells are more or less graceful, having long, turreted spires and small apertures. The color of the shells is generally a uniform greenish or yellowish, although some species have color bands, and the aperture is frequently tinged with purple or reddish.
The animal is very interesting in captivity. It is not very bold and will lie on the bottom of an aquarium with its head and foot half protruding from its shell, and its rostrum and tentacles slowly moving about. Frequently it may be seen moving along with its head and rostrum bent down and moving about like a hound on the scent.
A family closely allied to the last is the Melaniidae, the animals of which inhabit the entire world, except North America. They may be distinguished from the last family by the presence of little finger-like digitations on the edge of the mantle. The shells are generally larger and more highly colored than those of the last family, many of them being of a dark chocolate color and some are of a beautiful glossy black; some shells are smooth, while others are ornamented by knobs and spines. The genus Melania, a species of which is illustrated on the plate accompanying this article, is the most characteristic form.
The largest and handsomest of the fresh-water snails belong to the two families Viviparidae and Ampullariidae, the shells of the latter family frequently attaining a length of three inches. The animals of the first family prefer a sandy beach in a large lake or river, while those of the second generally live in more or less muddy rivers, ponds and creeks. A single genus of Viviparidae (Campelona) is confined solely to the United States, east of the Rocky Mountains. Their shells are generally of a rich grass green and in certain localities they may be collected by the thousands. Unlike many of the snails of which we have been writing, this family is viviparous, that is, brings forth its young alive, instead of laying eggs, as in the family Limnaeidae. This character has given the family its name, which is certainly well chosen. When born the shell is about one-sixteenth of an inch in length and is perfectly transparent. The animal is very active and eats voraciously of any vegetation within reach. Another handsome shell belonging to this group is the Vivipara contectoides, which is about an inch in length and is encircled by several color bands. It is a common shell in many of our ponds.
Somewhat larger and handsomer than the Viviparas are the Ampullarias, or apple-shells (also called idol-shells and pond snails). These animals live mostly in tropical and subtropical regions and are noted for the tenacity with which they retain their hold on life. So tenacious of life are they that instances are known of their living for several years away from the water, in this respect resembling some of the land snails. It is also recorded that hollow pieces of logwood from Honduras have frequently contained specimens of this family alive after a journey of thousands of miles. They may be said to be truly amphibious.
The writer has collected in Florida the large Ampullaria depressa in considerable numbers. It was noted particularly that this species furnished the principal food of the Everglade Kite, a bird inhabiting the southern part of Florida. Large quantities of these shells were found about the nesting places of these birds, from which the animal had been neatly extracted without damaging the shell in the least. The bird is, curiously enough, provided with a curved bill which easily fits into the aperture of the mollusk and extracts the animal with little difficulty, and the feet and claws are so constructed that the shell may be firmly held during the operation. This shell is figured on the plate.
In Central Africa there is a lake, Tanganyika, having a length of four hundred miles and a width of from ten to fifty miles, and at an elevation of twenty-seven hundred feet above sea level, which has one of the most interesting and peculiar fresh-water molluscan faunas known. It is thought that at some remote period in geological history this lake formed a part of the ocean and that in the course of time it was cut off from the sea, gradually became fresh and was finally raised to its present elevation. The reason for such a theory is the presence in the lake of certain molluscan organisms whose shells closely resemble those of the salt water family, Littorinidae (Periwinkles). The fact that certain species of the family inhabit brackish water and are even subject to the influence of fresh water, adds additional weight to this theory. The shell of this species (Limnotrochus thomasi) also resembles certain of the top-shells (Trochus), which are marine in habitat. Most of the other species inhabiting this lake are like the fresh water Viviparas in form.
The animal of Ampullaria depressa is very curious and interesting when studied alive. The foot is very wide, almost square in some positions; the head is narrow, separated from the body by a neck and the region of the mouth is produced into two long, cylindrical, tapering, tentacular processes, which are probably tactile organs like the elongated lips of Glandina, described in the last article. On the top of the head are placed the two whip-like tentacles, which are longer than the length of the whole animal and are always waving about when the animal is in motion. Just back of the tentacles the eyes are placed at the end of two short, rounded prominences or peduncles. From the left edge of the aperture extends the long, hollow, cylindrical siphon formed by two extensions of the mantle. On the upper side of the posterior end of the foot is placed the horny, concentric operculum or door. When the animal withdraws into its shell the head first disappears with its appendages and the siphon, and the foot is doubled up in the middle, the operculum shutting in last and closing the interior against all enemies.
All of the different groups of the mollusca have their giants and their pigmies and the fresh-water mollusks are no exception to the rule. We have thus far studied the animals of normal size and the giants. Let us now turn our attention to some of the pigmies among the fresh-water snails. One of the commonest of these small mollusks is the Bythinia tentaculata, the shell of which does not exceed half an inch in length, and is formed in a graceful, tapering turret. This species, like many other European animals, has been introduced into this country and bids fair to eclipse many of the native species in the number of individuals. It probably first came over with some merchandise, which was shipped west by the way of the Erie canal. The snail, once established in the canal, has had every opportunity to spread over the entire United States. The canal is emptied every year and cleaned and the water, with its organisms, is allowed to flow into the little streams and the larger rivers and thence into Lake Ontario. From this lake this species has spread so that it is also found in Lakes Erie and Michigan, and will eventually spread over the entire northern portion of the United States. This is but one of the many examples of different species being carried by human agencies from one part of the world to another.
But there are many species of these smaller fresh-water snails that are pigmies, indeed, whose tiny shells do not exceed an eight of an inch in length and which require the aid of a microscope to adequately study their delicate organisms. These minute animals live on water plants and on any submerged object. They vary from long, pointed, steeple-like shells to those which are perfectly rounded like a miniature apple. In our own country these little creatures may be found in any of our ponds or streams, and the lively little animals are well worth a closer acquaintance. They are known scientifically under the difficult names of Paludinella, Amnicola, Somatogyrus, Fluminicola, with many others, and do not bear any specific English titles.